1. Technical Field
The present invention relates generally to very sensitive thermometric instruments, known as microbolometers, which are used for the detection and measurement of radiant energy. More specifically, the present invention addresses correction of microbolometer output.
2. Related Art
Infrared detectors known as microbolometers respond to impinging infrared radiation through subtle variations in the temperature of the detector element. The detector elements include a material having a high temperature coefficient of resistance (TCR) such that these subtle variations in the temperature of the detector may be sensed. The sensing methods often employed are based on the passing of a metered electrical current through the device and measuring the resulting voltage drop. Alternatively, the temperature of the detector may be sensed by biasing the detector circuit with a known voltage and measuring the resulting current. In the simplest embodiment, the microbolometer detector is connected to a meter, and the response of the meter can be correlated to the intensity of the impinging infrared radiation.
However, in typical applications for which an image is desired, a lens is employed to focus energy onto a two-dimensional array of microbolometer detectors such that a spatially varying infrared field can be detected and converted to visible imagery using electronics and display means such as are commonly employed for visible imagery using Charge-Coupled Device (CCD) cameras. The electronics typically include a multiplexing circuit in intimate contact with the microbolometer array which converts the voltage or current variation of the many microbolometer elements to one or several multiplexed analog (e.g., voltage variation) data streams. This analog data is then converted to digital data using an analog-to-digital converter (ADC), and is then further processed to produce data for analysis or imagery on a Cathode Ray Tube (CRT) or similar video monitor.
The fact that the detection means is based on the thermal variations of the detector causes several practical problems. First, the material must be thermally isolated from surrounding matter so that a sufficiently large (e.g., several mK) temperature variation may occur as a result of the weak impinging infrared energy. Liddiard, in U.S. Pat. Nos. 4,574,263 and 5,369,280, and Higashi, et al., in U.S. Pat. No. 5,300,915 describe a microbolometer that provides thermal isolation by depositing a semiconductor material onto a pellicle, or xe2x80x9cmicro-bridgexe2x80x9d structure that physically separates the detector from the supporting substrate. Second, the temperature of the supporting substrate must be stable so that erroneous signals are not generated from its temperature fluctuations. Experience indicates that a 15 mK variation of substrate temperature within the sampling period (or video frame rate, whichever is greater) is acceptable, but fluctuations greater than this present a significant source of system noise. Third, the output of the microbolometer varies as a result of both the impinging infrared radiation, and the absolute temperature of the substrate. In this last case, the array output may be higher or lower at different temperatures, even if that temperature is held to within the stability requirement of 15 mK. Fourth, variations in the physical construction of the microbolometer detectors result in significant variations of the output of individual microbolometer detectors within the array, and these non-uniformities must be corrected in order to obtain a low-noise image.
As a result, there exists a need for an apparatus capable of correcting the output of a microbolometer, for example, in a focal plane array (FPA), such that the effects of thermal drift are removed or eliminated.
In the particular problem of thermal variation of the substrate, microbolometer detectors are operated at a fixed temperature, typically with a stability tolerance of xc2x10.015xc2x0 C. (i.e., 15 mK). Peltier-junction heat engines and control circuitry are commonly employed for this purpose. While this temperature stabilization scheme works well, it is not the ideal solution. For instance, the temperature stabilization system represents a significant portion of the detector package cost. Further, it is susceptible to damage from shock or vibration, and ordinarily requires tens of seconds to reach operational temperature from system start-up. Also, the temperature stabilization means is a major consumer of system power.
It is an advantage of the present invention to provide a system and method for correction of microbolometer output. For example, the present invention provides a method to eliminate the need for gross temperature stabilization of a microbolometer through the creation of a system that uses electronic means to correct the temperature variation of the microbolometer. An advantage of the present invention is that it eliminates the need for recalibration of a microbolometer appliance, for instance a microbolometer camera, should the temperature of the focal plane array in the camera change from the temperature for which it was calibrated. Further, rapid system readiness is possible since thermal stabilization of the focal plane array is not necessary. Specifically, this invention conditions the multiplexed output of a microbolometer focal plane array so that the peak-to-peak voltage of the analog signal is within the range of an analog-to-digital converter""s input sensitivity at any arbitrary temperature between approximately xe2x88x9210xc2x0 C. and 50xc2x0 C.
A further general aspect of this invention is to provide a method of correcting the output of a microbolometer, comprising: providing a temperature stabilization system for correcting the temperature variation of the microbolometer; providing an electronic system for conditioning the output of the microbolometer; and conditioning the output signal of the microbolometer.
A third general aspect of this invention is to provide a device for correction of microbolometer output, said device comprising: at least one microbolometer detector; an electrical reference circuit connected to said at least one microbolometer detector; an output from said electrical reference circuit connected to an input of a signal conditioning circuit; and an output from said signal conditioning circuit connected to a display device.
A fourth general aspect of this invention is to provide a microbolometer correction circuit comprising: at least one microbolometer detector; an analog-to-digital converter for converting the output of said microbolometer detector into a digital signal; a signal processor programmed to apply an algorithm to said digital signal to generate a correction signal; a memory device for storing said correction signal; and an electrical circuit for combining said correction signal with the output of said microbolometer detector.
A fifth general aspect of this invention is to provide a method for correction of the output of a microbolometer detection circuit comprising: providing at least one microbolometer detector, said microbolometer detector connected to a reference circuit; applying the output of said reference circuit to a signal processing system; providing a temperature sensor operationally connected to said at least one microbolometer detector; sensing the temperature of said microbolometer detector and producing a sensed temperature signal; applying the sensed temperature signal to said signal processing system; calculating a correction factor based on the outputs of said reference circuit and said sensed temperature signal; combining said correction factor with said reference circuit output to produce a corrected signal; and outputting said corrected signal.
A sixth general aspect of this invention is to provide a method for correction of the output of a microbolometer detection circuit comprising: providing a microbolometer detector circuit; converting the output of said microbolometer detector circuit into a digital signal; applying an algorithm to said digital signal; generating a correction signal; storing said correction signal; and applying said correction to the microbolometer cell.
A seventh general aspect of this invention is to provide a method for correction of the output of a focal plane array of microbolometer detectors employing continuous stabilization of the focal plane array temperature, said method comprising: providing a system for thermal stabilization of the focal plane array; sensing the focal plane array temperature; applying a correction algorithm to correct the analog output of the focal plane array to bring the peak-to-peak value of the analog output within a sensitivity range of an analog-to-digital converter; applying the calculated offset values to the first analog frame signals produced by the focal plane array to produce a resultant signal; converting the resultant signal to digital data; applying previously stored gain values to produce a corrected image signal; and outputting the corrected image signal.
An eighth general aspect of this invention is to provide a method for correction of the output of a focal plane array of microbolometer detectors employing passive stabilization of the focal plane array temperature, said method comprising: providing a thermal mass upon which is mounted the focal plane array; sensing a first focal plane array temperature; applying a correction algorithm to correct the analog output of the focal plane array to bring the value of the analog output signal within a sensitivity range of an analog-to-digital converter; applying the calculated offset values to the first analog frame signals produced by the focal plane array to produce a resultant signal; converting the resultant signal to digital data; applying previously stored gain values to the digital data to produce a corrected image signal; and outputting the corrected image signal.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of embodiments of the invention.